1. Field of the Invention
The present invention relates to a resin encapsulation type semiconductor device, in which a semiconductor element is encapsulated with an epoxy resin composition.
2. Description of the Related Art
A novolak type epoxy resin composition using, for example, a phenolic novolak resin as a curing agent is excellent in its moldability. In addition, the composition is excellent in molding characteristics and the cured product of that is excellent in its resistance to humidity (i.e., resistance to moisture) and electrical characteristics under high temperatures. Thus, the composition of this type is widely used for encapsulating a semiconductor device.
In recent years, miniaturization of each functioning member of a semiconductor element and enlargement of the element itself are being rapidly promoted in the field of a resin encapsulation type semiconductor device in accordance with progress in the integration density of the semiconductor element. As a result, required are the mounting of the element at a high density and automation of the assembling process. Under the circumstances, the mounting process of a semiconductor device is being changed from the conventional pin insertion type to a surface mounting type.
The surface mounting method for mounting a semiconductor device to a substrate is applied to, for example, the mounting of a surface mounting type package such as a gate array called ASIC (Application Specific IC) or a standard cell type LSI. In this case, a cream solder on a substrate is heated by an infrared ray or a fluorocarbon vapor for connection to a lead of a semiconductor device. In the heating step, the entire package is exposed to such a high temperature as about 215.degree. to 260.degree. C. The rapid heating tends to cause crack occurrence in the package. To be more specific, since the resin which has absorbed moisture is exposed to a high temperature, the water within the package is evaporated, leading to swelling of the package. Further, the stress accompanying the swelling causes the resin to be cracked. Where the cracking reaches the outer surface of the resin (package), the reliability in terms of resistance to humidity is markedly impaired. What should also be noted is that the resin itself is also swollen, making it impossible to achieve the mounting. Still further, cracking also occurs in a PSG (phosphosilicate glass) layer or a SiN (silicon nitride) layer used as a passivation film on a wiring layer of aluminum. Alternatively, a gold bonding wire tends to be broken in some cases.
As described above, the reliability of the device is markedly lowered by the crack occurrence. Presently, how to prevent the crack occurrence is a serious problem to be solved in the surface mounting process.
As a measure for solving the problem, demands are being raised for further improving the properties of the encapsulating resin. Particularly, when it comes to a resin encapsulation type semiconductor device of a large package, it is demanded that the stress imparted by the encapsulating resin to the encapsulated material be diminished, and that the bonding strength be increased between the encapsulating resin and the PSG, SiN, and polyimide films on the surface of the semiconductor element and between the encapsulating resin and the lead frame. It is also demanded that the encapsulating resin be imparted with sufficient strength and humidity resistance under high temperatures to which the package is exposed in the mounting step, and that the water vapor absorption amount of the encapsulating resin be diminished.
In view of these demands, a resin composition containing a polyfunctional epoxy resin having a heat-resistant skeletal structure and/or a polyfunctional phenolic resin curing agent is proposed recently as an encapsulating resin having a strength high enough to withstand the water vapor generated within the package. However, the semiconductor device encapsulated with such a heat-resistant encapsulating resin (composition) is unsatisfactory in reliability in terms of humidity resistance and in resistance to external contamination, compared with the device encapsulated with a novolak type epoxy resin composition. Also, the above heat-resistant encapsulating resin is poor in mold release characteristics. It is certainly possible to improve the mold release characteristics by adding a sufficient amount of a mold release agent. In this case, however, the bonding strength is lowered between the cured resin and the semiconductor element and between the cured resin and the lead frame. It follows that, in the resin encapsulation type semiconductor device using the above encapsulating resin, the package tends to be cracked in the mounting step, leading to deterioration in the reliability in terms of humidity resistance.